Plastic fuel tanks intended for motor vehicles have to meet specifications that specify maximum permissible amplitudes of deformation on their lower and upper skins. The deformations stated in these specifications usually have to be met during aging test in which the tank contains a certain quantity of fuel for a given period of time (typically several weeks) and at a given temperature (usually 40° C.). The purpose of these specifications is to ensure that vehicles maintain their road clearance and to prevent the skin of the tank from coming into contact with hot spots of the vehicle and with the chassis.
At the present time, plastic fuel tanks are generally fixed to the chassis of the vehicle via plastic lags eventually supported by metal straps. Additionally a few contact points with the car chassis are foreseen. The latter are used in particular on the larger tanks where compliance with maximum permissible deformations is more difficult. However, recourse to these straps involves an additional attachment step and is therefore not very economical.
Recently, a new category of vehicle has been introduced to the market, which uses both electricity and internal combustion to propel itself. This group of vehicles has been called “hybrid” vehicles. Although these vehicles make up only a small portion of the global automotive market, their market share increases each year. A new derivation of the hybrid uses electricity only for the first 40 to 60 miles of a given journey assuming the vehicle was plugged into electrical power for a predetermined amount of time before the journey. These vehicles are deemed “plug-in hybrids”.
Typically, fuel vapors are generated inside of a fuel tank due to fuel pressure and temperature variations and are stored in a charcoal canister to prevent evaporative emissions of hydrocarbons into the atmosphere. These vapors are periodically purged out of the canister and sent to the engine where they are consumed during the normal combustion process. On a standard gasoline engine vehicle this can occur whenever possible to prevent the canister from becoming stuffed and bleeding hydrocarbons into the environment. Generally these purging periods and associated purge volumes are limited on a hybrid vehicle and when the vehicle is operating in electric mode no purging at all can occur. A “plug-in hybrid” vehicle may go many driving circles without ever running the gasoline engine. Therefore, a need arises for the fuel system to contain vapor for long periods of time by keeping the system sealed and under pressure in order to limit fuel evaporation.
There are several solutions to limit the loading with vapors of the carbon canister. One of these solutions is to seal the tank. This will pressurize the tank because vapor generation is highly related to the pressure inside the fuel tank. Vapor formation leads to a build-up of a pressure up to a certain equilibrium point where basically no more vapor is formed. It is generally assumed that no more vapor generation occurs after a pressure of 30 to 45 kPa has been built up. Thus, a tank pressurized with pressure from approximately 20 kPa to approximately 50 kPa will significantly reduce the loading with fuel vapors of the carbon canister.
The presently used plastic fuel tanks are generally not designed for an internal pressure above 10 kPa without showing a significant deformation. However, the specifications to be met for deformation of the tank walls are very narrow, so that it is important to avoid an increase in deformation.
A well known technique to improve the deformation stability of hollow plastic bodies is to use a so-called kiss point or tack-off point, like described in U.S. 2002/0100759. The principle of this technique is to locally connect the upper and lower walls of the tank through weld points/areas. The main draw back of this technique is that since the kiss points are of small size in order to limit the loss of useful volume of the hollow body, they lead to the concentration of mechanical stress at aid kiss points, which may lead to cracks or other damages over time.
Solutions have been proposed in the prior art to enhance the mechanical strength of fuel tanks.
Thus, it has been proposed in U.S. Pat. Nos. 3,919,373 and 4,891,000 that an insert be secured inside the tank at the time of its blow-molding from a cylindrical peruse. However, it is not easy to position the insert accurately using this technique, which makes the blow-molding process more difficult and more costly and time consuming.
U.S. Pat. No. 5,398,839 discloses a fuel tank with an outer and an inner shell where the inner shell is divided by intersecting internal walls inside the inner shell. The intersecting walls are integrally molded with undercut wall openings which allow communication of liquid and vapor between the compartments. The internal walls have to be fixed to the upper and lower walls of the tank by a suitable welding technique during the molding process. This requires a very complicated design of the molding process and it is basically impossible to produce such products by molding.
Another technique to increase the deformation stability of the fuel tank is to use means of contention around such tank. If the means are metallic the weight of the fuel system increases significantly and if the means are made of the lighter structures, the structure itself occupies a lot of space and thus again reduces the useful volume of the tank.